1. Field of the Invention
The present invention relates to methods and systems for relay transmission utilizing a relay transmitter located between data transmitters on the transmission and reception sides and, more specifically, to a method and a system for relay transmission in which data transmission is not done concurrently with data reception.
2. Description of the Background Art
In order to improve throughput of packet data transmission from a transmitter on the transmission side to a transmitter on the reception side via a relay transmitter, a technology disclosed in Japanese Patent Laid-Open Publication No. 9-181772 (97-181772) may be a possibility. This conventional technology is described next below.
FIG. 15 is a diagram showing the structure of a data transmission system in the above conventional technology. In FIG. 15, the data transmission system includes a data terminal 1521 where data is generated, a data transmitter on the transmission side 1511 for transmitting the received data in the form of packet, a relay transmitter 1512 for receiving the packet data from the data transmitter on the transmission side 1511 via a low-orbit satellite circuit 1503, a relay transmitter 1513 for receiving the packet data from the relay transmitter 1512 via a stationary-satellite circuit 1504, a data transmitter on the reception side 1514 for receiving the packet data from the relay transmitter 1513 via a radio circuit 1505 being a cellular network, and a data terminal 1522 for receiving the data from the data transmitter on the reception side 1514 for data processing.
Described next is the operation of the data transmission system. The data transmitted from the data terminal 1521 is provided with an error detection code by the data transmitter on the transmission side 1511 for transmission in the form of packet. The data packet is relayed via the relay transmitters 1512 and 1513 to the data transmitter on the reception side 1514.
If any packet erroneously disappears during such relay transmission on a data link, its disappearance is informed by the to-be-transmitted next packet. Once detected such packet disappearance, the relay transmitter transmits a retransmission request to the transmitter preceding thereto. The requested transmitter then retransmits a packet identical to the disappeared.
In such conventional data transmission system, data transmission and reception can be simultaneously done since transmission media on one data link are independent of each other. Accordingly, the retransmission request with an arbitrary timing from the relay transmitter causes no influence on data transmission carried out on other data links.
Here, in simultaneous data transmission and reception by radio, a radio wave is generally intensely emitted by a transmitter for radio transmission but is weakly received because it has traveled across the long distance. Consequently, the radio wave during transmission interferes with that during reception, rendering data reception difficult. This is an exemplary near-far problem. Moreover, if a transmission channel and a reception channel are close in frequency to each other in such case, interference therebetween is increased due to leakage of signal component from the transmission channel, rendering data reception more difficult. If those channels are equal in frequency, needless to say, the difficulty for data reception gets considerably high. Thus, transmitting the retransmission request in the conventional data transmission system may affect data transmission on other data links, resulting in transmission failure. In order to avoid such a problem, a carefully designed circuit for assuredly receiving data is required, although leading to high cost and another effort to design and manufacture a device equipped with such a circuit.
To reduce cost and size of the transmitters, equipping each transmitter with only one radio modem is effective. Generally, the radio modem, however, is functionally incapable of simultaneously performing data transmission and reception even in the different frequencies, and switching between the frequencies must be concurrently done. Consequently, if those transmitters share the same frequency of channel, the retransmission request transmitted in the conventional manner may collide with data being transmitted on other data links, resulting in transmission failure. Even if the transmitters do not share the same frequency of channel, the radio modems should be switched between frequencies with appropriate timing, otherwise failed in relay.
Therefore, an object of the present invention is to provide a method and a system for relay transmission being free from data collision with a transmission request on data links and accordingly causing no failure in relay.
The present invention has the following features to attain the object above.
A first aspect of the present invention is directed to a relay transmission method for not-concurrently performing data transmission and reception, and sequentially transmitting a data frame from a transmitter on the transmission side to a transmitter on the reception side via one or more relay transmitters, the method comprises: a data transmission step of plurally transmitting the data frame with a frame number added from the transmitter on the transmission side to the transmitter on the reception side; a data transfer step of sequentially transferring the data frame by one or more relay transmitters; a retransmission request transmission step of transmitting, to the transmitter on the transmission side, a retransmission request indicating any data frame, by frame number, which has not normally received by the transmitter on the reception side; a retransmission request transfer step of sequentially transferring the retransmission request by one or more relay transmitters; a data retransmission step of sequentially retransmitting the data frame requested by the retransmission request from the transmitter on the transmission side to the transmitter on the reception side; and a retransmission data transfer step of sequentially receiving and transferring the data frame by one or more relay transmitters.
As described above, in the first aspect, even if the transmitters share the same frequency, data transmission or retransmission request on a data link does not affect that being carried out on another data link, thereby preventing data collision. Further, by sequentially transferring data, data included in a control frame such as retransmission request frame is proportionally increased. Therefore, data transfer can be efficiently done. Still further, by shortening a frame, retransmission can be efficiently achieved even on a radio transmission path where an error rate is high.
According to a second aspect, further to the first aspect, the data transfer step comprises the steps of: storing, in a buffer each provided in one or more relay transmitters, any data frame normally received thereby; and sequentially transferring the normally-received data frame. Further, the retransmission request transfer step comprises the steps of: deleting, from the retransmission request received by one or more relay transmitters, any frame number corresponding to the data frame stored in the buffer therein; and sequentially transferring the retransmission request after deletion of numbers. Still further, the retransmission data transfer step comprises the steps of: adding, to the data frame received by one or more relay transmitters, any data frame stored in the buffer; and sequentially transferring the data frame added with a newly-generated data frame.
As described above, in the second aspect, the number of data frames retransmitted from the transmission on the transmission side can be reduced, thereby shortening time for retransmission and thus increasing the efficiency of retransmission.
According to a third aspect, further to the second aspect, the retransmission request transfer step further comprises the steps of: determining whether the buffer stores every frame number included in the retransmission request received by one or more relay transmitters; and retransmitting, if determined every frame number included, the data frame requested by the retransmission request to the transmitter on the reception side, and if determined not included, deleting the frame number found in the buffer from the retransmission request for sequential transfer.
As described above, in the third aspect, any wasteful retransmission request is prevented.
According to a fourth aspect, further to the first aspect, one or more relay transmitters have a directional antenna which is initially adjusted in a predetermined direction, wherein the data transfer step, the retransmission request transfer steps. Further, the retransmission data transfer step comprise the steps of: setting, at data reception, a retention time Ta for the directional antenna to be in the predetermined direction by one or more relay transmitters; adjusting the directional antenna in a direction where an adjacent transmitter on transmission side is located by one or more relay transmitters; and readjusting, with a lapse of the retention time Ta, the directional antenna back in the initially predetermined direction by one or more relay transmitters.
As described above, in the fourth aspect, in a case where a transmission path is a radio transmission path, a directional antenna is used and changed its direction depending on from which data comes. In this manner, a relay transmitter by radio can be directionally set for transmission/reception so that multipath fading can be prevented. Further, the antenna can be adjusted to be ready for next transmission frame without waiting, i.e., no longer than required, for transmission frames which have not transmitted due to some error. Therefore, data transfer can be assuredly achieved.
According to a fifth aspect, further to the fourth aspect, the directional antenna is plurally provided; and the directional antennas are switched there among for setting direction thereof.
As described above, in the fifth aspect, a directional antenna is used and changed its direction depending on from which data comes. In this manner, a relay transmitter by radio can be directionally set for transmission/reception so that multipath fading can be efficiently prevented.
According to a sixth aspect, further to the first aspect,
in one or more relay transmitters, a predetermined frequency channel is initially selected among plural, and
the data transfer step, the retransmission request transfer step, and the retransmission data transfer step comprise the steps of:
setting, at data reception, a retention time Ta to be in the predetermined frequency channel by one or more relay transmitters; selecting a frequency channel for communicating with the transmitter on the transmission side by one or more relay transmitters; and selecting again, with a lapse of the retention time Ta, the predetermined initial frequency channel by one or more relay transmitters.
As described above, in the sixth aspect, each different frequency channel is available for transmitters. Therefore, even if any disturbance wave source is observed in the vicinity of the transmitters, and even if such source has each different frequency characteristics, the transmitters can be in each appropriate frequency channel. Accordingly, a throughput can be increased.
According to a seventh aspect, further to the sixth aspect, selection of the frequency channel is based on which frequency channel shows a maximum throughput with the transmitter on the transmission side.
As described above, in the seventh aspect, a throughput can be maximized by selecting the frequency channel appropriately for each communications section.
According to an eighth aspect, further to the first aspect, the data frame and the retransmission request are each additionally provided with transfer valid period information indicating a predetermined time left for transfer. Further, the data transmission step comprising the steps of: calculating, by sequentially deducting a time taken for transmitting one data frame from the predetermined time, a time left for transfer; and adding the calculated time left for transfer as another transfer valid period information. The data transfer step, the retransmission request transmission step, the retransmission request transfer step, the data retransmission step, and the retransmission data transfer step comprise the steps of
calculating, by sequentially deducting a time taken between data reception and transmission from the transfer valid period information included in the data frame or the retransmission request, the time left for transfer; adding the calculated time left for transfer as another transfer valid period information; and stopping data transmission or transfer when the another transfer valid period information becomes negative.
As described above, in the eighth aspect, each transmitter can know a time left for transfer on the path, preventing data transfer from being carried out after a transfer valid period.
According to a ninth aspect, further to the eighth aspect, the transfer valid period information is a natural number N when the time taken for transmitting one frame is presumed to be 1.
As described above, in the ninth aspect, the efficiency of data transfer can be improved by reducing an area of the transfer valid period information added to the transmission frame.
According to a tenth aspect, further to the eighth aspect, one or more relay transmitters each have a directional antenna which is initially adjusted in a predetermined direction. The data transfer step, the retransmission request transfer step, and the retransmission data transfer step comprise the steps of:
setting a retention time Ta for the directional antenna to be in the predetermined direction by one or more relay transmitters using the transfer valid period information; adjusting the directional antenna in a direction where the transmitter on transmission side is located by one or more relay transmitters; and readjusting, with a lapse of the retention time Ta, the directional antenna back in the initially predetermined direction by one or more relay transmitters.
As described above, in the tenth aspect, in a case where a transmission path is a radio transmission path, a directional antenna is used and changes its direction depending on from which data comes. In this manner, a relay transmitter by radio can be directionally set for transmission/reception so that multipath fading can be prevented. Further, the antenna can be adjusted to be ready for next transmission frame without waiting, i.e., no longer than required, for transmission frames which have not transmitted due to some error. Therefore, data transfer can be assuredly achieved.
According to an eleventh aspect, further to the eighth aspect, in one or more relay transmitters, a predetermined frequency channel is initially selected among plural. The data transfer step, the retransmission request transfer step, and the retransmission data transfer step comprise the steps of: setting, at data reception, a retention time Ta to be in the predetermined channel by one or more relay transmitters; selecting a frequency channel for communicating with the transmitter on the transmission side by one or more relay transmitters; and selecting again, with a lapse of the retention time Ta, the predetermined initial frequency channel by one or more relay transmitters.
As described above, in the eleventh aspect, when a transmission path is a radio transmission path, each different frequency channel is available for transmitters. Therefore, even if any disturbance wave source is observed in the vicinity of the transmitters, and even if such source has each different frequency characteristics, the transmitters can be in each appropriate frequency channel. Accordingly, a throughput can be increased. Further, the frequency channel can be set to be ready for next transmission frame without waiting, i.e., no longer than required, for transmission frames which have not transmitted due to some error. Therefore, data transfer can be assuredly achieved. According to a twelfth aspect, further to the first aspect, the retransmission request transmission step comprising the steps of: comparing a predetermined time allowed for the transmitter on the transmission side to start next transmission with a maximum length of time taken to retransmit data responding to the retransmission request, and not transmitting the retransmission request when the predetermined time is smaller in value, and when the predetermined time is equal or larger, transmitting a retransmission request indicating any data frame, by frame number, which has not normally received by the transmitter on the reception side to the transmitter on the transmission side.
As described above, in the twelfth aspect, data collision can be successfully prevented, especially data collision often observed between a current transmission and the following transmission when data transmission is repeated with a predetermined intervals to attain a constant throughput. This is occurred because the number of retransmission cannot be specified depending on in which state the transmission path is.
According to a thirteenth aspect, further to the twelfth aspect, the maximum length of time taken to retransmit data responding to the retransmission request is obtained by adding 1 to the number of to-be-retransmitted frames, multiplying the value by the number of transfers and a time taken to transmit one frame, and adding the value with xcex1 (a predetermined value determined by processing capability).
As described above, in the thirteenth aspect, a manner to calculate the maximum length of time taken for retransmission in the twelfth aspect is typically specified. In this manner, the transmitter on the reception side can easily determine whether to transmit a retransmission request. Further, data collision can be prevented between a current transmission and the following transmission. Still further, the calculation done in the transmitter on the reception side can be simplified.
A fourteenth aspect of the present invention is directed to a relay transmission system for non-concurrently performing data transmission and reception, and sequentially transmitting a data frame from a transmitter on the transmission side to a transmitter on the reception side via one or more relay transmitters. The transmitter on the transmission side in accordance with the fourteenth aspect of the present invention comprises: a reception part for receiving a retransmission request indicating any data frame, by frame number, which has not normally received by the transmitter on the reception side; a frame header analysis part for extracting a frame number from the retransmission request received by the reception part; a data frame generation part for generating a retransmission data frame by adding transmission data with the frame number; and a transmission part for transmitting the transmission data frame. The transmitter on the reception side in accordance with the fourteenth aspect of the present invention comprises: a reception part for receiving the data frame; a frame header analysis part for extracting the frame number from the data frame received by the reception part; a retransmission request frame generation part for generating a retransmission request including the frame number of the data frame not normally received by the transmitter on the reception side; and a transmission part for transmitting the retransmission request. One or more relay transmitters in accordance with the fourteenth aspect of the present invention each comprises: a reception part for receiving the data frame or the retransmission request; and a transmission part for transmitting the data frame or the retransmission request received by the reception part. When the transmitter on the reception side fails to receive every data frame transmitted by the transmitter on the transmission side, the transmitter on the reception side transmits the retransmission request, and responding thereto, the transmitter on the transmission side sequentially retransmits the data frame to the transmitter on the reception side.
As described above, in the fourteenth aspect, even if the transmitters share the same frequency, data transmission or retransmission request on a data link does not affect that being carried out on another data link, thereby preventing data collision. Further, by sequentially transferring data, data included in a control frame such as retransmission request frame is proportionally increased. Therefore, data transfer can be efficiently done. Still further, by shortening a frame, retransmission can be efficiently achieved even on a radio transmission path where an error rate is high.
According to a fifteenth aspect, further to the fourteenth aspect, one or more relay transmitters each further comprises: a retransmission frame buffer for storing any normally-received data frame, and generating a transmission data frame, in response to retransmission of data frame by the transmitter on the transmission side, a transmission data frame by adding the stored data frame to the data frame; and a retransmission request frame reconstruction part for deleting, from the retransmission request, any frame number corresponding to the data frame stored in the retransmission frame buffer.
As described above, in the fifteenth aspect, the number of data frames retransmitted from the transmission on the transmission side can be reduced, thereby shortening time for retransmission and thus increasing the efficiency of retransmission.
According to a sixteenth aspect, further to the fourteenth aspect, one or more relay transmitters further comprise: a directional antenna for transmitting/receiving a signal; and an antenna control part for controlling the directional antenna to direct in a direction where the signal goes/comes, and readjusting the directional antenna in a predetermined initial direction with a lapse of a predetermined retention time Ta.
As described above, in the sixteenth aspect, in a case where a transmission path is a radio transmission path, a directional antenna is used and changed its direction depending on from which data comes. In this manner, a relay transmitter by radio can be directionally set for transmission/reception so that multipath fading can be prevented. Further, the antenna can be adjusted to be ready for next transmission frame without waiting, i.e., no longer than required, for transmission frames which have not transmitted due to some error. Therefore, data transfer can be assuredly achieved.
According to a seventeenth aspect, further to the fourteenth aspect, one or more relay transmitters each further comprise: a directional antenna for transmitting/receiving a signal; an antenna switching part for selectively switching among plurally-provided the directional antennas; and
an antenna control part for controlling the antenna switching part to direct the directional antennas in a direction where the signal goes/comes, and with a lapse of a predetermined retention time Ta, readjusting the directional antennas in the predetermined initial direction.
As described above, in the seventeenth aspect, a directional antenna is used and changed its direction depending on from which data comes. In this manner, a relay transmitter by radio can be directionally set for transmission/reception so that multipath fading can be efficiently prevented.
According to an eighteenth aspect, further to the fourteenth aspect, one or more relay transmitters each further comprise a frequency channel control part for selecting a frequency channel among plural, and with a lapse of a predetermined retention time Ta set to be in the selected frequency channel, reselecting the same frequency channel, and the transmission part and the reception part transmit/receive a signal in the frequency channel selected by the frequency channel control part.
As described above, in the eighteenth aspect, when a transmission path is a radio transmission path, each different frequency channel is available for transmitters. Therefore, even if any disturbance wave source is observed in the vicinity of the transmitters, and even if such source has each different frequency characteristics, the transmitters can be in each appropriate frequency channel. Accordingly, a throughput can be increased. Further, the frequency channel can be set to be ready for next transmission frame without waiting, i.e., no longer than required, for transmission frames which have not transmitted due to some error. Therefore, data transfer can be assuredly achieved.
According to a nineteenth aspect, further to the fourteenth aspect, the transmitter on the transmission side further comprises: a transfer valid period control part for calculating a time left for transfer, as another transfer valid period information, by sequentially deducting a time taken for transmitting one data frame from transfer valid period information indicating a predetermined time allowed for the transfer, and stopping transmission when the transfer valid period information becomes negative, and a transfer valid period addition part for adding the transfer valid period information to the data frame. The transmitter on the reception side, in accordance with the nineteenth aspect, comprises: a transfer valid period control part for calculating a time left for transfer, as another transfer valid period information, by sequentially deducting a time taken for transmitting one data frame from transfer valid period information indicating a predetermined time allowed for the transfer, and stopping transmission when the transfer valid period information becomes negative, and a transfer valid period addition part for adding the transfer valid period information to the retransmission request. One or more relay transmitters, in accordance with the nineteenth aspect, each further comprises: a transfer valid period control part for calculating a time left for transfer, as another transfer valid period information, by sequentially deducting a time taken between data reception and transmission from the transfer valid period information included in the data frame or the retransmission request, and stopping transmission when the transfer valid period information becomes negative, and a transfer valid period addition part for adding the transfer valid period information to the data frame or the retransmission request.
As described above, in the nineteenth aspect, each transmitter can know a time left for transfer on the path, preventing data transfer from being carried out after a transfer valid period.
According to a twentieth aspect, further to the nineteenth aspect, the transfer valid period information is a natural number N when the time taken for transmitting one frame is presumed to be 1.
According to a twenty-first aspect, further to the nineteenth aspect, one or more relay transmitters each further comprise: a directional antenna for transmitting/receiving a signal; and an antenna control part for controlling the directional antenna to direct in a direction where the signal goes/comes, and readjusting the directional antenna in a predetermined initial direction with a lapse of a predetermined retention time Ta determined by using the transfer valid period information.
As described above, in the twenty-first aspect, in a case where a transmission path is a radio transmission path, a directional antenna is used and changed its direction depending on from which data comes. In this manner, a relay transmitter by radio can be directionally set for transmission/reception so that multipath fading can be prevented. Further, the antenna can be adjusted to be ready for next transmission frame without waiting, i.e., no longer than required, for transmission frames which have not transmitted due to some error. Therefore, data transfer can be assuredly achieved.
According to a twenty-second aspect, further to the nineteenth aspect, one or more relay transmitters each further comprise a frequency channel control part for selecting a frequency channel among plural to receive/transmit a signal, and with a lapse of a predetermined retention time Ta determined using the transfer valid period information, reselecting the same frequency channel, and the transmission part and the reception part transmit/receive a signal in the frequency channel selected by the frequency channel control part.
As described above, in the twenty-second aspect, when a transmission path is a radio transmission path, each different frequency channel is available for transmitters. Therefore, even if any disturbance wave source is observed in the vicinity of the transmitters, and even if such source has each different frequency characteristics, the transmitters can be in each appropriate frequency channel. Accordingly, a throughput can be increased. Further, the frequency channel can be set to be ready for next transmission frame without waiting, i.e., no longer than required, for transmission frames which have not transmitted due to some error. Therefore, data transfer can be assuredly achieved. A twenty-third aspect of the present invention is directed to a transmitter on the transmission side for non-concurrently performing data transmission and reception, and sequentially transmitting a data frame to a transmitter on the reception side via one or more relay transmitters. A transmitter in accordance with the twenty-third aspect of the present invention comprises: a reception part for receiving a retransmission request indicating any data frame, by frame number, which has not normally received by the transmitter on the reception side; a frame header analysis part for extracting a frame number from the retransmission request received by the reception part; a data frame generation part for generating a retransmission data frame by adding transmission data with the frame number; and a transmission part for transmitting the transmission data frames. When the transmitter on the reception side fails to receive every data frame transmitted, the transmitter on the reception side transmits the retransmission request, and responding thereto, the transmitter on the transmission side sequentially retransmits the data frame to the transmitter on the reception side.
As described above, in the twenty-third aspect, even if the transmitters share the same frequency, data transmission or retransmission request on a data link does not affect that being carried out on another data link, thereby preventing data collision. Further, by sequentially transferring data, data included in a control frame such as retransmission request frame is proportionally increased. Therefore, data transfer can be efficiently done. Still further, by shortening a frame, retransmission can be efficiently achieved even on a radio transmission path where an error rate is high.
According to a twenty-fourth aspect, the twenty-third aspect, the transmitter on the transmission side further comprises: a transfer valid period control part for calculating a time left for transfer, as another transfer valid period information, by sequentially deducting a time taken for transmitting one data frame from transfer valid period information indicating a predetermined time allowed for the transfer, and stopping transmission when the transfer valid period information becomes negative, and a transfer valid period addition part for adding the transfer valid period information to the data frame.
As described above, in the twenty-fourth aspect, the number of data frames retransmitted from the transmission on the transmission side can be reduced, thereby shortening time for retransmission and thus increasing the efficiency of retransmission.
A twenty-fifth aspect of the present invention is directed to a transmitter on the reception side for non-concurrently performing data transmission and reception, and sequentially transmitting a data frame to a transmitter on the transmission side via one or more relay transmitters. A transmitter in accordance with the twenty-fifth aspect of the present invention comprises: a reception part for receiving the data frame; a frame header analysis part for extracting the frame number from the data frame received by the reception part; a retransmission request frame generation part for generating a retransmission request including the frame number of the data frame not normally received by the transmitter on the reception side; and a transmission part for transmitting the retransmission request. When failed to receive every data frame transmitted from the transmitter on the transmission side, transmits the retransmission request, and responding thereto, the transmitter on the transmission side sequentially retransmits the data frame.
As described above, in the twenty-fifth aspect, even if the transmitters share the same frequency, data transmission or retransmission request on a data link does not affect that being carried out on another data link, thereby preventing data collision. Further, by sequentially transferring data, data included in a control frame such as retransmission request frame is proportionally increased. Therefore, data transfer can be efficiently done. Still further, by shortening a frame, retransmission can be efficiently achieved even on a radio transmission path where an error rate is high.
According to a twenty-sixth aspect, further to the twenty-fifth aspect the transmitter on the reception side further comprises: a transfer valid period control part for calculating a time left for transfer, as another transfer valid period information, by sequentially deducting a time taken for transmitting one data frame from transfer valid period information indicating a predetermined time allowed for the transfer, and stopping transmission when the transfer valid period information becomes negative, and a transfer valid period addition part for adding the transfer valid period information to the retransmission request.
As described above, in the twenty-sixth aspect, the number of data frames retransmitted from the transmission on the transmission side can be reduced, thereby shortening time for retransmission and thus increasing the efficiency of retransmission.
A twenty-seventh aspect of the present invention is directed to one or more relay transmitters for non-concurrently performing data transmission and reception, and sequentially transmitting a data frame from a transmitter on the transmission side to a transmitter on the reception Sides. Each relay transmitter in accordance with the twenty-seventh aspect of the present invention comprises: a reception part for receiving the data frame or the retransmission request; and a transmission part for transmitting the data frame or the retransmission request received by the reception part. When the transmitter on the reception side failed to receive every data frame transmitted by the transmitter on the transmission side, the transmitter on the reception side transmits the retransmission request, and responding thereto, the transmitter on the transmission side sequentially retransmits the data frame to the transmitter on the reception side.
As described above, in the twenty-seventh aspect, even if the transmitters share the same frequency, data transmission or retransmission request on a data link does not affect that being carried out on another data link, thereby preventing data collision. Further, by sequentially transferring data, data included in a control frame such as retransmission request frame is proportionally increased. Therefore, data transfer can be efficiently done. Still further, by shortening a frame, retransmission can be efficiently achieved even on a radio transmission path where an error rate is high.
According to a twenty-eighth aspect, further to the twenty-seventh aspect, one or more relay transmitters further comprises: a transfer valid period control part for calculating a time left for transfer, as another transfer valid period information, by sequentially deducting a time taken between data reception and transmission from the transfer valid period information included in the data frame or the retransmission request, and stopping transmission when the transfer valid period information becomes negative, and a transfer valid period addition part for adding the transfer valid period information to the data frame or the retransmission request.
As described above, in the twenty-eighth aspect, the number of data frames retransmitted from the transmission on the transmission side can be reduced, thereby shortening time for retransmission and thus increasing the efficiency of retransmission.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.